Often, single or multiple layers of differing compositions must be applied to a substrate. For example, in the manufacture of photographic film as many as twelve layers of differing compositions must be applied in a distinct layered relationship. Close tolerances on uniformity are required. The use of sequential coating operations can produce a plurality of distinct superposed layers on a substrate, or all of the layers can be simultaneously applied in one station. In using coating technology it is desirable to produce layers that are no thicker than is necessary to achieve a desired function. Indeed, a prime motivation for simultaneous multilayer coating is that by grouping layers together in a composite the individual layers may be so thin that they are impossible to coat as individual layers. Also, thicker wet coatings would increase the material cost of the products. Similarly, it is desirable to reduce the amount of solvent in coating fluid formulations. While solvents and diluents make formulations easier to process by lowering viscosity and increasing the bulk volume, their cost and the cost of safely disposing of them is undesirable.
One important style of coating die popular in the photographic industry is the slide coater. U.S. Pat. No. 2,761,419 teaches its use for multilayer coating. This coating die is also useful for thin single layer coating. FIG. 1 illustrates the features of a multilayer coating die 10'. This die has three plates 12, 14, 16 separated by fluid distribution slots 18, 20 arranged so that the fluids exit from the slots onto incline planes 22, 24 and flow down them. At the termination of the plane 24, the coating fluid is transferred from the die lip 26 across a small gap to a moving substrate 28.
Slide curtain coating is disclosed in U.S. Pat. No. 3,632,403. At the end of the incline plane of the slide die, the fluid is allowed to separate and fall by gravity as a sheet before contacting the moving substrate. FIG. 2 illustrates such coating die. An improvement on this is its use for simultaneous multilayer curtain coating. U.S. Pat. No. 3,508,947 teaches this method for coating photographic elements. Still another style of slide curtain die is shown in the Japanese application 51-39264 where the orientation of the slot and inclines onto which the coatings exit are inverted with respect to gravity.
In coating operations, coating dies often become contaminated with low surface energy materials. This may cause coating defects and dramatically raise the probability of producing scrap material. The production of coated products of reactive or curing coating fluids often requires frequent cleaning of the slide die surfaces to avoid unwanted encrustations of gelled material. Cleaning can be facilitated by covering the die surfaces with lower energy release materials such as silicones or polytetrafluoroethylene. It is therefore desirable to modify the coating dies to allow coating when the surfaces have low surface energies.